Memory device and method of manufacturing the same

ABSTRACT

Disclosed is a memory device provided with a plurality of memory cells and a lead-out line ( 12 ) shared among the memory cells. Each memory cell is provided with a transistor ( 6 ) formed above a substrate ( 1 ) and a variable resistance element ( 10 ) having a lower electrode ( 7 ), an upper electrode ( 9 ) that comprises a noble metal, and a variable resistance layer ( 8 ) disposed between the lower electrode ( 7 ) and the upper electrode ( 9 ). The resistance value of the variable resistance layer ( 8 ) changes reversibly in response to electric pulses that go through the transistor ( 6 ) and are applied between the lower electrode ( 7 ) and the upper electrode ( 9 ). The lead-out line ( 12 ) is in direct contact with the upper electrodes ( 9 ) of the memory cells.

TECHNICAL FIELD

The present invention relates to a memory device having memory cellseach of which includes a transistor and a variable resistance elementwhose resistance value changes reversibly based on electrical signals,and a method of manufacturing such a memory device.

BACKGROUND ART

Recent years have seen advancement in the research and development ofmemory devices having memory cells with variable resistance elements. Avariable resistance element is an element with a property in which itsvariable resistance value changes reversibly based on electric signals,and can store data corresponding to the resistance value in anonvolatile manner.

Commonly known as a memory device using variable resistance elements isa memory device which includes memory cells, known as a 1T1R (onetransistor one resistor) memory cells, arranged in a matrix at positionsat which bit lines cross with word lines and source lines that arearranged orthogonally to the bit lines. Here, the 1T1R memory cellsrefer to memory cells each of which is formed by connecting in series ametal oxide semiconductor (MOS) transistor and a variable resistanceelement (see PTL 1 through 3 for example).

Disclosed in Patent Literature (PTL) 1 is a memory device including 1T1Rmemory cells in which oxides having a perovskite crystal structure areused as variable resistance elements.

FIG. 10 is a diagrammatic cross-sectional view of a conventional memorycell described in PTL 1. A memory cell 1011 includes a select transistor1006 and a variable resistance element 1010. The transistor 1006includes, on a semiconductor substrate 1001, a source region 1002serving as a first diffusion layer region, a drain region 1003 servingas a second diffusion layer region, and a gate electrode 1005 formed ona gate oxide film 1004. The variable resistance element 1010 is formedby having a variable resistance layer 1008 disposed between a lowerelectrode 1007 and an upper electrode 1009. Here, the variableresistance layer 1008 is a layer whose resistance value changes inresponse to an applied voltage.

The drain region 1003 and the lower electrode 1007 are electricallyconnected and connected in series via a conductive via 1050 a. The upperelectrode 1009 is connected to a metal line 1012 serving as a bit linevia a conductive via 1050 b, and the source region 1002 is connected toa metal line 1013 serving as a source line via a conductive via 1050 c.Moreover, the gate electrode 1005 is connected to a word line (not shownin the drawings). It is to be noted that PTL 1 discloses Pr₁ _(—)_(x)Ca_(x)MnO₃ (PCMO), La₁ _(—) _(x)Sr_(x)MnO₃ (LSMO) and so on as thematerials of the variable resistance layer 1008.

Furthermore, in the memory cell 1011 constituted as describe above,application of a pulse voltage Vpp to the upper electrode 1009, a pulsevoltage Vss to the source region 1002, and a pulse voltage having apredetermined voltage amplitude Vwp to the gate electrode changes theresistance state of the variable resistance layer 1008 from a lowresistance state to a high resistance state. Here, Vpp is a voltageapplied by a voltage switch circuit (not shown in the drawings) forwriting or erasing data, Vss is a ground voltage, and Vwe is a voltageat or above a threshold value which changes the resistance state of thevariable resistance element 1010 from a high resistance state to a lowresistance state. Inversely, application of the pulse voltage Vss to theupper electrode 1009, the pulse voltage Vpp to the source region 1002,and a pulse voltage having a predetermined voltage amplitude Vwe to thegate electrode changes the resistance state of the variable resistancelayer 1008 from a high resistance state to a low resistance state.

Similarly, disclosed in PTL 2 is a memory device including 1T1R memorycells using a tantalum oxide that is a binary oxide of a transitionmetal and oxygen as a variable resistance element.

Moreover, recent studies regarding variable resistance elements havefound that by using a two-layered stacked structure for the variableresistance layer included in the variable resistance element, a morepreferable characteristic with respect to the variable resistanceelement can be obtained. For example, as disclosed in PTL 3, byconfiguring the variable resistance layer as a stacked structurecomprising a tantalum oxide having a composition represented as TaO_(x)(where 0<x<2.5) and a tantalum oxide having a composition represented asTaO_(y) (where x<y<2.5), a forming procedure becomes unnecessary, and avariable resistance element exhibiting a high-speed, stable, reversiblerewrite characteristic and a favorable resistance value retentioncharacteristic can be realized.

CITATION LIST Patent Literature

[PTL 1]

-   Japanese Unexamined Patent Application Publication No. 2005-25914    [PTL 2]-   WO 2008/59701    [PTL 3]-   WO 2008/149484

SUMMARY OF INVENTION Technical Problem

It is common to use a transition metal oxide such as PCMO or TaO_(x)(where x>o) for the variable resistance layer included in the variableresistance element, and common to use a noble metal such as platinum(Pt) as an electrode material. However, various problems arise whenconfiguring a memory device by integrating a variable resistance elementcomprising a material such as these on what is known as a siliconsemiconductor.

One of the problems stems from the poor adhesion of the electrodematerial used in the variable resistance element (especially noblemetals such as Pt) to the insulating layer (usually silicon (SiO₂) isused). For example, in FIG. 10, a gap is created between the insulatinglayer 1051 and the surface and walls of the upper electrode 1009 thatthe insulating layer 1051 (usually SiO₂) covers due to the poor adhesionof the electrode material such as Pt and the SiO₂ included in theinsulating layer 1051. Accordingly, there is a concern that the metalmaterial used to form the conductive via 1050 b will leak out from thegap when the conductive via 1050 b corresponding to the upper electrode1009 is formed. Specifically, when tungsten (W) is used as the metalmaterial to form the conductive via 1050 b, for example, tungstenhexafluoride (WF₆), which is used as the metal material gas, leaks outfrom the gap between the upper electrode 1009 and the insulating layer1051, and W forms covering the upper electrode 1009. As a result, ashape abnormality is formed (that is, a malformation) at the interfacebetween the upper electrode 1009 and the conductive via 1050 b. Thisshape abnormality causes an abnormality of a characteristic with respectto the variable resistance element 1010, and is unfavorable with respectto reliability of the line connected to the upper electrode 1009. Inother words, it is believed that fluctuation of the electricalcharacteristic of the initial resistance value and such of the variableresistance element is due to the shape abnormality formed at theinterface between the upper electrode 1009 and the conductive via 1050b.

FIG. 11 is a cross-sectional SEM photograph of a variable resistanceelement experimentally manufactured using Pt in the upper electrode andSiO₂ in the insulating layer. In normal circumstances, W is only formedinside a contact hole provided for forming the conductive via. However,in FIG. 11, W leaks out from the contact hole for the conductive via andcovers a portion of the upper electrode at the interface between theupper electrode and the conductive via (the portion inside the whitedashed line labeled with a W in FIG. 11). As a result, the formation ofa shape abnormality can be confirmed at the interface between the upperelectrode 1009 and the conductive via 1050 b.

Moreover, usually, to form a conductive via corresponding to an upperelectrode of a variable resistance element, an insulating layer (usuallySiO₂) which covers the upper electrode is formed, then a contact holewhich penetrates through the insulating layer reaching the upperelectrode is formed using a common dry etching method, and a metalmaterial (usually W) is filled inside the contact hole by CVD. Here,during the process of dry etching which forms the contact hole, dryetching is performed by over etching, whereby the upper electrodeexposed in the contact hole is slightly etched so etching residue fromthe insulating layer is not generated. As a result, the film thicknessof the portion of the upper electrode on the bottom of the formedcontact hole which was etched is thicker than the film thickness of theportion of the upper electrode which was not etched. In other words, aconcave shape is formed on the upper electrode. It is to be noted that,in this circumstance, because dry etching using a higher density plasmais used when the contact hole is miniaturized, the formation of theconcave shape on the upper electrode is prone to occur to a higherdegree in the over etching process.

Thus, in a variable resistance element in which a conductive via isformed, the occurrence of film thickness unevenness of the upperelectrode at the time the conductive via is formed is unavoidable. It isbelieved that this unevenness in film thickness of the upper electrodeleads to a fluctuation in the electrical characteristic of the initialresistance value and such of the variable resistance element. This pointwill be discussed in further detail afterwards.

Upon creation of the present invention, the inventors newly discoveredthat the initial resistance value of the variable resistance element isdependent on the film thickness of the upper electrode included in thevariable resistance element. This point will be explained hereinafter.

FIG. 12 is a graph showing initial resistance values of a variableresistance element including an upper electrode comprising Pt, avariable resistance layer comprising TaO_(x), and a lower electrodecomprising tantalum nitride (TaN) when the film thickness of the Ptincluded in the upper electrode is 5 nanometers (nm), 10 nm, and 20 nm.

In FIG. 12, for each Pt film thickness, initial resistance values forthree types of variable resistance elements are shown for when the areaof the upper electrode is 0.5 square micrometers (μm²), 1 μm², and 5μm². Approximately 40 elements having these three types of electrodeswere experimentally manufactured and measured for initial resistancevalue. The median values are shown in FIG. 12. It is to be noted thatwhen the test elements were made, before depositing the upper electrodeon the variable resistance layer, an oxygen plasma treatment isadministered to a surface of the variable resistance layer using anashing device, whereby the variable resistance layer is formed asstacked structure including a tantalum oxide having a compositionrepresented as TaO_(x) (where 0<x<2.5) and a thickness of 23 nm, and atantalum oxide having a composition represented as TaO_(y) (where x<y)and a thickness of 8 nm, as is shown in PTL 3.

A trend can be interpreted from FIG. 12 in which the initial resistancevalue of the variable resistance element increases as the layerthickness of the upper electrode comprising Pt is made thinner.

When Pt is used in the upper electrode, a micro-protrusion forms in adownward facing direction in post processing after the formation of theupper electrode. FIG. 13 is a cross-sectional TEM photograph of avariable resistance element comprising Pt after undergoing this type ofpost processing. The shape of the micro-protrusion formed on the upperelectrode protruding in the direction of the variable resistance layer(here TaO_(x) is used) can be seen in the dashed-line circle in thisphotograph. This is a change the Pt layer undergoes during a heatingprocess which is part of the post process procedure. In other words,this is a process which causes the Pt atoms to migrate toward theTaO_(y) when the Pt layer is heated to a high temperature. The bulk ofthe protrusion can be seen to be growing from the area of the grainboundaries of the Pt layer. This thought to be due to the propensity formigration to occur along the grain boundaries of the Pt layer.

As a result of further examination, the inventors discovered that thegrowth of this protrusion is dependent on the film thickness of theupper electrode, and that the growth of the protrusion is suppressedwhen the film thickness of the upper electrode is thin. With respect toa special characteristic of the variable resistance element, it isbelieved the overall resistance value of the variable resistance layerdecreases when the protrusion is present in comparison to when theprotrusion is not present due to the film thickness of the variableresistance layer becoming substantially thin in the portion of theprotrusion. In FIG. 12, when the thickness of the Pt included in theupper electrode is 5 nm, the initial resistance value of the variableresistance layer increases, and when the thickness of the Pt is 20 nm,the initial resistance value of the variable resistance layer decreases,reflecting the inclination described above.

The fluctuation of the initial resistance value of the Pt with athickness of 10 nm in FIG. 12 is markedly greater than the other samplesbecause the film thickness of Pt in the vicinity of 10 nm reaches athreshold level for the generation of the protrusion described above. Inother words, when the Pt film thickness is approximately 10 nm, thegeneration of the protrusion is inconsistent, occurring in someinstances and not in others. This inconsistency is believed to be thecause of the fluctuation seen in the initial resistance value. The samephenomenon can also be observed in materials other than Pt such aspalladium (Pd). Moreover, this phenomenon can also be observed in thevariable resistance layer with compositions such as HfO_(x) or ZrO_(x)in addition to TaO_(x).

Considering how this dependency affects the characteristics of an actualvariable resistance element, the occurrence of film thickness unevenness(due to the film thickness of the upper electrode exposed in the contacthole being thinner than in the other ones) of the upper electrode at thetime the contact hole is formed is unavoidable in a variable resistanceelement in which a conductive via is formed. Consequently, thefluctuation of the initial resistance value of the variable resistanceelement is also unavoidable, which is undesirable.

As described above, as a result of the shape abnormality formed at theinterface between the upper electrode and the conductive via, as well asthe film thickness unevenness of the upper electrode as a result of overetching at the time the contact hole is formed in order to form theconductive via, an electrical characteristic of the variable resistanceelement such as the initial resistance value also becomes unstable.

Accordingly, the object of the present invention is to solve theproblems outlined above by providing a memory device having variableresistance elements in which initial resistance value fluctuations aresuppressed by improving the physical connection shape between anelectrically connected a line and an upper electrode of the variableresistance elements, and by reducing the film thickness unevenness ofthe upper electrode.

Solution to Problem

In order to solve the problems outlined above, the memory deviceaccording to an embodiment of the present invention comprises aplurality of memory cells and a lead-out line provided in common to theplurality of memory cells, wherein each of the memory cells includes atransistor formed above a substrate and a variable resistance elementwhich includes a lower electrode, an upper electrode comprising a noblemetal, and a variable resistance layer disposed between the lowerelectrode and the upper electrode, the variable resistance layer havinga resistance value that changes reversibly in response to an electricpulse that go through the transistor and are applied between the lowerelectrode and the upper electrode, and the lead-out line being in directcontact with the upper electrode included in each of the memory cells.

The lead-out line is a line formed by a first patterning of a conductivefilm. In other words, it is a line in contact with the upper electrodewithout the use of a via. Moreover, the bottommost surface of the bottomsurfaces of the lead-out line is formed to be lower than the top surfaceof the upper electrode.

With this configuration, a contact hole forming process (dry etchingprocess) can be omitted at the time of forming the conductive viabecause the line connected to the upper electrode included in thevariable resistance element is in direct contact with the upperelectrode without the disposition of the conductive via. Consequently,film thickness unevenness of the upper electrode that occurs duringcontact hole formation is avoided. As a result, fluctuation of theinitial resistance value of the variable resistance element thatoriginates from the film thickness unevenness of the upper electrode canbe reduced. Moreover, because the upper electrode included in thevariable resistance element is in direct contact with the lead-out line,the contact area of the upper electrode and the insulating layerdecreases, and a gap will not form between the upper electrode and theinsulating layer. Additionally, because a conductive via connected tothe upper electrode is not formed, the leaking of metal material(usually W) used to form the conductive via is avoided, and the physicalconnection shape (contact shape) between the electrically connected theline and the upper electrode included in the variable resistance elementcan be improved.

Moreover, when an electrical connection via a conductive via is used,like in the conventional configuration shown in FIG. 10, the amount ofcurrent that can flow to the variable resistance element is limited bythe amount of current that can flow to the conductive via (which dependson the cross-sectional area of the conductive via and theelectromigration property of the material used in the conductive via).On the other hand, with the configuration described above, theelectrical connection portion between the upper electrode and thelead-out line can be increased to the size of the area of the upperelectrode. Supposing that a current with a current density comparable towhen a conductive via is used can flow, one can deduce that an amount ofcurrent that can flow to the variable resistance element is sufficientlygreater than in the conventional configuration. Additionally, when asuitable material is selected for the lead-out line, a sufficientlygreat current can flow to the variable resistance element, and thedegree of freedom can be increased with respect to the selection ofmaterial used in the variable resistance element (in particular thevariable resistance layer) and to the operation setting values of thevariable resistance element (and the memory device).

Moreover, it is preferable that (i) the variable resistance layerinclude a first layer comprising a first oxygen-deficient transitionmetal oxide having a composition represented as MO_(x) (where M denotesa transition metal and O denotes oxygen) and a second layer comprising asecond oxygen-deficient transition metal oxide having a compositionrepresented as MO_(y) (where x<y) formed on the first layer, and (ii)the second layer be in contact with the upper electrode.

When tantalum oxide is used, the variable resistance layer is formedhaving a stacked structure comprising a tantalum oxide having acomposition represented as TaO_(x) (where 0<x<2.5) and a tantalum oxidehaving a composition represented as TaO_(y) (where x<2.5). As a result,a forming procedure becomes unnecessary, and a variable resistanceelement exhibiting a high-speed, stable, reversible rewritecharacteristic and a favorable resistance value retention characteristiccan be realized. Consequently, a memory device exhibiting favorablecharacteristics can be realized.

Moreover, it is preferable that at least a portion of a side wall of thevariable resistance layer be covered by an insulating layer.

With this configuration, at least a portion of the side walls of thevariable resistance layer are covered by the insulating layer. Forexample, because the portion of the variable resistance layer in contactwith an interlayer insulating layer is strongly oxidized during theformation of the interlayer insulating layer, the resistance of the sidewalls of the variable resistance layer increases (the variableresistance layer becomes insulative). For this reason, even if thevariable resistance layer is exposed in the etch-back process andbrought into contact with the lead-out line, current will not flow fromthe portion in contact. Consequently, with this configuration, theetch-back margin can be increased in the process exposing the upperelectrode at the time the lead-out line is formed.

Moreover, it is preferable that the variable resistance layer comprisean oxygen-deficient transition metal oxide selected from the groupconsisting of tantalum (Ta), hafnium (Hf), and zirconium (Zr).

With this configuration, when the variable resistance layer comprises anoxide such as TaO_(x) (where 0<x<2.5), HfO_(x) (where 0<x<2.0) andHfO_(y), or ZrO_(x) (where 0<x<2.0), the portion of the variableresistance layer in contact with the interlayer insulating layer isstrongly oxidized to a further degree in the process depositing theinterlayer insulating layer to cover the variable resistance element. Inother words, the resistance of the side walls of the interlayerinsulating layer increases (the interlayer insulating layer becomesinsulative). Thus, current can only flow to the variable resistancelayer through the upper electrode and the lower electrode.

Moreover, it is preferable that the lead-out line have a stackedstructure comprising an electrically conductive material, and a portionof the lead-out line that is in direct contact with the upper electrodecomprises an electrically conductive material including at least one of:chromium (Cr), molybdenum (Mo), niobium (Nb), tantalum (Ta), tungsten(W), titanium (Ti), vanadium (V), zirconium (Zr), hafnium (Hf), silicon(Si), nitrogen (N), carbon (C), and boron (B).

With this configuration, by using a material having a highelectromigration hardness such as the transition metals Cr, Mo, Nb, Ta,W, Ti, V, Zr, or Hf, the chemical compounds silicide, nitride, carbide,or boride in the line which is in direct contact with the upperelectrode included in the variable resistance element, a sufficientlygreat current can flow to the variable resistance element. Accordingly,the degree of freedom can be increased with respect to the selection ofmaterial used in the variable resistance element and to the operationsetting values of the variable resistance element (and the memorydevice). Furthermore, having a layer comprising a transition metal suchas those described above or a chemical compound such as silicide,nitride, carbide, or boride and deposited between the line and theinsulating layer and between the upper electrode and line isadvantageous in that the layer also functions as an adhesion layer.

Moreover, a method of manufacturing a memory device according to anaspect of the present invention comprises: forming the transistor abovea substrate; forming a lower electrode of the variable resistanceelement to correspond to the transistor; forming a variable resistancelayer on the lower electrode; forming an upper electrode comprising anoble metal on the variable resistance layer; forming an interlayerinsulating layer so as to cover the variable resistance element;exposing at least a portion of a top surface of the upper electrode; andforming a lead-out line that is provided in common to the plurality ofmemory cells so as to be in direct contact with the upper electrode.

With this configuration, film thickness unevenness of the upperelectrode that occurs during contact hole formation is avoided becausethe line connected to the upper electrode included in the variableresistance element is in direct contact with the upper electrode withoutthe disposition of the conductive via. As a result, fluctuation of theinitial resistance value of the variable resistance element that resultsfrom the film thickness unevenness of the upper electrode can bereduced. Moreover, because the upper electrode included in the variableresistance element is in direct contact with the lead-out line, thecontact area of the upper electrode and the insulating layer decreases,and a gap will not form between the upper electrode and the insulatinglayer. Additionally, because a conductive via connected to the upperelectrode is not formed, the leaking of metal material (usually W) usedto form the conductive via can be avoided, and the contact shape can beimproved.

Moreover, when an electrical connection via a conductive via is used,the amount of current that can flow to the variable resistance elementis limited by the amount of current that can flow to the conductive via(which depends on the cross-sectional area of the conductive via and theelectromigration property of the material used in the conductive via).On the other hand, with the configuration described above, theelectrical connection portion between the upper electrode and thelead-out line can be increased to the size of the area of the upperelectrode. Supposing that a current with a current density comparable towhen a conductive via is used can flow, one can deduce that an amount ofcurrent that can flow to the variable resistance element is sufficientlygreater than in the conventional configuration. Additionally, when asuitable material is selected for the lead-out line, a sufficientlygreat current can flow to the variable resistance element, and thedegree of freedom can be increased with respect to the selection ofmaterial used in the variable resistance element (in particular thevariable resistance layer) and to the operation setting values of thevariable resistance element (and the memory device). Furthermore,because a photomask which defines the conductive via connected to theupper electrode is unnecessary, the number of photomasks used can bedecreased by one layer, thereby shortening the manufacturing process.

Moreover, it is preferable that (i) forming, in the variable resistancelayer, a first layer comprising an first oxygen-deficient transitionmetal oxide having a composition represented as MO_(x) (where M denotesa transition metal and O denotes oxygen), and (ii) forming, on the firstlayer, a second layer comprising a second oxygen-deficient transitionmetal oxide having a composition represented as MO_(y) (where x<y), beincluded.

When tantalum oxide is used, the variable resistance layer is formedhaving a stacked structure comprising a tantalum oxide having acomposition represented as TaOx (where 0<x<2.5) and a tantalum oxidehaving a composition represented as TaOy (where x<2.5). As a result, aforming procedure becomes unnecessary, and a variable resistance elementexhibiting a high-speed, stable, reversible rewrite characteristic and afavorable resistance value retention characteristic can be realized.Consequently, a memory device exhibiting favorable characteristics canbe manufactured.

Moreover, it is preferable that the exposing (i) include planarizing theinterlayer insulating layer, and (ii) exposes at least a portion of theupper electrode on a surface of the interlayer insulating layer afterplanarizing the interlayer insulating layer.

With this configuration, in the process planarizing the interlayerinsulating layer, damage to the upper electrode can be prevented whileat the same time exposing the upper electrode.

Moreover, it is preferable that in the forming of a lead-out line, thelead-out line and an other line be formed at the same time.

With this configuration, the process of manufacturing the line can besimplified due to the lead-out line and the other line being formed atthe same time.

Advantageous Effects of Invention

With the present invention, it is possible to provide a memory deviceand a method of manufacturing the memory device having variableresistance elements in which initial resistance value fluctuations aresuppressed by improving the physical connection shape between anelectrically connected a line and an upper electrode of the variableresistance elements, and by reducing the film thickness unevenness ofthe upper electrode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagrammatic view of the memory device according to thefirst embodiment, including: (a) a cross-sectional view showing aconfiguration of the memory device; (b) a cross-sectional view as viewedfrom the direction of the arrow along the line A-A′ shown in (a); and(c) a plain view as viewed from the surface of (a).

FIG. 2 is a process drawing showing an example of the method ofmanufacturing the memory device according to the first embodiment,including: (a) a drawing showing a process for stacking material for thelower electrode, the variable resistance layer, and the upper electrodeabove the first insulating layer; (b) a drawing showing a process forforming the variable resistance element on the first insulating layer;and (c) a drawing showing a process for forming the interlayerinsulating layer to cover the variable resistance element.

FIG. 3 is a process drawing showing an example of the method ofmanufacturing the memory device according to the first embodiment,including: (a) a drawing showing a process for planarizing the surfaceof the interlayer insulating layer; (b) a drawing showing a process forexposing the upper electrode included in the variable resistanceelement; (c) a drawing showing a process for forming the lead-out linewhich is in direct contact with the upper electrode included in thevariable resistance element: and (d) a drawing showing a process forforming the metal line which is connected to the conductive via.

FIG. 4 is a block diagram showing a configuration of the nonvolatilememory device according to the first embodiment.

FIG. 5 is a cross-sectional SEM photograph of a variable resistanceelement experimentally manufactured using the method of manufacturingthe memory device according to the first embodiment.

FIG. 6 is a diagrammatic view of a memory device according to the secondembodiment, including: (a) a cross-sectional view showing aconfiguration of the memory device; (b) a cross-sectional view as viewedfrom the direction of the arrow along the line B-B′ shown in (a); and(c) a plain view as viewed from the surface of (a).

FIG. 7 is a process drawing showing an example of the method ofmanufacturing the memory device according to the second embodiment,including: (a) a drawing showing a process for planarizing the surfaceof the interlayer insulating layer; (b) a drawing showing a process forforming the conductive via which is connected to the metal line; (c) adrawing showing a process for exposing the upper electrode included inthe variable resistance element; and (d) a drawing showing a process forforming the metal line which is connected to the conductive via at thesame time as an electrode line which is in direct contact with the upperelectrode.

FIG. 8 is a diagrammatic view of a memory device according to the thirdembodiment including: (a) a cross-sectional view showing a configurationof the memory device; and (b) a cross-sectional view as viewed from thedirection of the arrow along the line C-C′ shown in (a).

FIG. 9 is a process drawing showing an example of the method ofmanufacturing the memory device according to the third embodiment inwhich the process of exposing the variable resistance layer and theupper electrode included in the variable resistance element is shown.

FIG. 10 is a diagrammatic cross-sectional view showing a configurationof a memory cell included in a conventional memory device.

FIG. 11 is a cross-sectional SEM photograph of a variable resistanceelement experimentally manufactured using Pt in the upper electrode andSiO₂ in the insulating layer.

FIG. 12 is a graph showing initial resistance values of a variableresistance element including an upper electrode comprising Pt, avariable resistance layer comprising TaO_(x), and a lower electrodecomprising tantalum nitride (TaN) when the film thickness of the Ptincluded in the upper electrode is 5 nm, 10 nm, and 20 nm.

FIG. 13 is a cross-sectional TEM photograph of the variable resistanceelement including Pt in the upper electrode and TaO_(x) in the variableresistance layer.

Hereinafter, embodiments of present invention are described withreference to the drawings. It is to be noted that the present inventionis explained with reference to the succeeding embodiments and drawings,but these are intended to be examples, and the present invention is notintended to be limited thereto.

(First embodiment)

[Configuration of the Memory Device]

FIG. 1 is a diagrammatic view of the memory device according to thefirst embodiment. FIG. 1 (a) is a cross-sectional view showing aconfiguration of the memory device, FIG. 1 (b) is a cross-sectional viewas viewed from the direction of the arrow along the line A-A′ shown inFIG. 1 (a), and FIG. 1 (c) is a plain view as viewed from the surface ofFIG. 1 (a). It is to be noted that in FIG. 1 (c), a number ofconstituents of the memory device which cannot be seen from the surfaceare shown with dashed lines and labeled with reference numerals.

A memory device 100 is provided with a plurality of 1T1R memory cells.Specifically, each of the memory cells is formed from a selecttransistor 6 and a variable resistance element 10 connected in serialvia a conductive via 50 a. The select transistor 6 includes,manufactured above a substrate 1 having a semiconductor, a source region2, a drain region 3, a gate oxide film 4, and a gate electrode 5 formedon the gate oxide film 4.

The variable resistance element 10 includes a variable resistance layer8 which is disposed between a lower electrode 7 and an upper electrode 9and whose resistance value changes in response to an applied voltage.Here, the lower electrode 7 and the drain region 3 are electricallyconnected via the conductive via 50 a. According to the firstembodiment, the variable resistance element 10 has a cylindrical shape.In other words, the variable resistance element 10 has a cylindricalshape when viewed from above, as is shown in FIG. 1 (c). It should benoted that the variable resistance element 10 is not limited to thisshape, and may be formed into a rectangular-parallelepiped-shape, forexample. Moreover, the gate electrode 5 is connected to a word line (notshown in the drawings).

Additionally, an interlayer insulating layer 15 is deposited in thevicinity of the variable resistance element 10. A lead-out line 12 (forexample a bit line) is deposited on the upper portion of the upperelectrode 9 included in the variable resistance element 10 so as tocover the upper electrode 9. A lead-out line is a line formed by a firstpatterning of a conductive film. In other words, it is a line in contactwith the upper electrode without the use of the via. As shown in FIG. 1(b), the lead-out line 12 is provided in common to a plurality of thememory cells. Moreover, the bottommost surface of the bottom surfaces ofthe lead-out line 12 is formed to be lower than the top surface of theupper electrode 9. In other words, the lead-out line 12 is formed tocover the entire surface as well as a portion of the side walls of theupper electrode 9, and furthermore is formed on the interlayerinsulating layer 15 within a predetermined range from the upperelectrode 9. It is to be noted that the lead-out line 12 may also beformed to cover a portion of the upper electrode 9 and not the entiretop surface of the upper electrode 9, and on a portion of the interlayerinsulating layer 15 within a predetermined range from the upperelectrode 9.

Here, when a predetermined electric pulse is applied to the variableresistance layer 8 included in the variable resistance element 10, theresistance state of the variable resistance layer 8 transitionsreversibly between a low resistance (LR) state (hereinafter referred toas LR state) and a high resistance (HR) state (hereinafter referred toas HR state). The resistance state of the variable resistance layer 8 ismaintained after a transition as long as the predetermined electricpulse is not applied. According to the first embodiment, the LR stateand the HR state are assigned with respective binary data (0 or 1), andelectric pulses having different polarities are applied in order tocause the resistance state of the variable resistance layer 8 totransition between the LR state and the HR state. A perovskite metaloxide, a transition metal oxide, or a portion of typical metal oxidescan be used as the variable resistance layer.

Specifically, the variable resistance layer 8 can comprise anoxygen-deficient TaO_(x) (where 0<x<2.5), HfO_(x) (where 0<x<2.0), orZrO_(x) (where 0<x<2.0), for example, and have a stacked structureincluding a high resistance layer and a low resistance layer. In thiscase, by regulating the thickness of the high resistance layer, what iscalled the forming process (a process applying an electric stimulationto the variable resistance element directly after manufacturing todevelop resistive state changes) becomes unnecessary, and it becomespossible to control the value of the current flowing in the variableresistance element. Details regarding relevant knowledge gained fromtest results can be found in, for example, PTL 3, and are thereforeomitted here.

Specifically, when TaO_(x) (where 0<x<2.5) is used in the variableresistance layer 8, it is preferable for the variable resistance layer 8to have a stacked structure including a first layer (low-concentrationoxide) comprising an oxygen-deficient tantalum oxide having acomposition represented as TaO_(x) (where 0.7≦x≦1.9), and a second layer(high-concentration oxide) comprising a second oxygen-deficient tantalumoxide having a composition represented as TaO_(y) (2.1≦y≦2.5). Here, theoxygen-deficient tantalum oxide is a tantalum oxide having anon-stoichiometric composition whose oxygen content (atomic ratio: ratioof number of oxygen atoms to total number of atoms) is less than atantalum oxide having a stoichiometric composition. When this stackedstructure is used, the second layer (high-concentration oxide layer) isdeposited so as to be in contact with the upper electrode 9.

With this configuration, the resistivity of the TaO_(x) decreases(becomes a low resistance layer), and the resistivity of the TaO_(y)increases (becomes a high resistance layer). By adopting thisconfiguration, the variable resistance element 10 exhibiting ahigh-speed, stable, reversible rewrite characteristic and a favorableresistance value retention characteristic can be realized. Detailsregarding relevant knowledge gained from test results can be found inPTL 3, and are therefore omitted here.

Moreover, the upper electrode 9 includes a noble metal. It is preferablethat one of a material or a plurality of materials having a higherstandard electrode potential than a standard electrode potential of atransition metal included in the variable resistance layer 8 be used asthe noble metal included in the upper electrode 9, and it is preferablethat a material having a lower standard electrode potential than thestandard electrode potential of a material included the upper electrode9 be used in the lower electrode 7 (for example W, Ni, or TaN). That is,when the variable resistance layer 8 comprises tantalum oxide and thestandard electrode potential of the lower electrode 7 is represented asV₁, the standard electrode potential of the tantalum as V_(Ta), and thestandard electrode potential of the upper electrode as V₂, it ispreferable that the difference of the standard electrode potential V₁ ofthe lower electrode 7 and the standard electrode potential V_(Ta) of thetantalum, represented as V₁−V_(Ta), and the difference of the standardelectrode potential V₂ of the upper electrode and the standard electrodepotential V_(Ta) of the tantalum, represented as V₂−V_(Ta), fulfill therelationship: 0<V₁−V_(Ta)<V₂−V_(Ta). Moreover, it is preferable that thedifference of the standard electrode potential V₁ of the lower electrode7 and the standard electrode potential V_(Ta) of the tantalum,represented as V₁−V_(Ta), and the difference of the standard electrodepotential V₂ of the upper electrode 9 and the standard electrodepotential V_(Ta) of the tantalum, represented as V₂−V_(Ta), fulfill therelationship: V₁−V_(Ta)≦0<V₂−V_(Ta).

By adopting such a configuration, a resistance changing phenomenon canoccur stably at the variable resistance layer 8 which is in contact withthe upper electrode 9.

Furthermore, a second insulating layer 16 comprising for example SiO₂ isdisposed on the lead-out line 12 and the interlayer insulating layer 15.A metal line 13 (for example a source line) is disposed on the secondinsulating layer 16. As shown in FIG. 1 (c), the metal line 13 isprovided in common to a plurality of the memory cells. The source region2 included in the select transistor 6 is connected, via a conductive via50 b, to a diffusion layer serving as a source line, or the metal line13. The gate electrode 5 of the select transistor 6 included in thememory cell is connected to the word line (not shown in the drawings),and the upper electrode 9 included in the variable resistance element 10or one of the diffusion layers of the transistor (for example, thesource region 2) is connected to a bit line (not shown in the drawings).It is to be noted that, as is shown in FIG. 4 (to be described later),the word lines and the bit lines are disposed so as to cross each other.A plurality of the memory cells are disposed in an array in thedirection of the word lines and the bit lines, forming a memory cellarray.

It is to be noted that the variable resistance layer 8 was exemplifiedas having a stacked structure of tantalum oxide, but the functionalitydescribed above is not limited to the use of a stacked structurecomprising tantalum oxide, and the present invention is not limited tothis configuration. For example, a configuration of a single layercomprising tantalum oxide is acceptable. Moreover, a single layercomprising a different transition metal such as hafnium (Hf) orzirconium (Zr) is also acceptable, as is a stacked structure comprisinghafnium oxide or a stacked structure comprising zirconium oxide. Thiswill be further discussed afterwards as configuration variations.

Moreover, the upper electrode 9 comprises, for example, one or aplurality of the following: gold (Au), platinum (Pt), iridium (Ir),palladium (Pd), silver (Ag), and copper (Cu), but is not limitedthereto. These materials with respect to the upper electrode 9 are givenas an example; other materials may be used.

As FIG. 1 shows, the memory device 100 according to the first embodimentis characterized by the lead-out line 12 serving as a bit line and beingdisposed so as to be in direct contact with the upper electrode 9,without the use of the conductive via 50 a and such. With thisconfiguration, a contact hole forming process (dry etching process) canbe omitted at the time of forming the conductive via, which inconventional technology is used to connect the upper electrode 9 and thelead-out line 12. As a result, film thickness unevenness of the upperelectrode 9 that occurs during contact hole formation is avoided, andfluctuation of the initial resistance value of the variable resistanceelement 10 that originates from the film thickness unevenness of theupper electrode is avoided.

Moreover, because a portion of the upper electrode 9 included in thevariable resistance element 10 is configured to be in direct contactwith the lead-out line 12 which is connected to the upper electrode 9,the contact area of the upper electrode 9 and the interlayer insulatinglayer 15 is decreased. As a result, the formation of a gap between theupper electrode 9 and the interlayer insulating layer 15 is markedlysuppressed. Furthermore, because a conductive via connected to the upperelectrode 9 is not formed, a contact shape malformation caused by theleaking of metal material (usually W) used to form the conductive viacan be avoided.

Furthermore, when an electrical connection via a conductive via is used,the amount of current that can flow to the variable resistance element10 is limited by the amount of current that can flow to the conductivevia (which depends on the cross-sectional area of the conductive via andthe electromigration property of the material used in the conductivevia). However, with the configuration according to the first embodiment,the area of the portion of the upper electrode 9 and the lead-out line12 that is electrically connected can be increased to the size of thearea of the upper electrode 9. Therefore, supposing that a current witha current density comparable to when a conductive via is used can flow,one can deduce that an amount of current that can flow to the variableresistance element is sufficiently greater than in the conventionalconfiguration.

Additionally, when a suitable material is selected for the lead-out line12, a sufficiently great current can flow to the variable resistanceelement 10, and the degree of freedom can be increased with respect tothe selection of material used in the variable resistance element 10 (inparticular the variable resistance layer 8) and to the operation settingvalues of the variable resistance element 10 (and the memory device 100)Moreover, advantageously, a photomask which defines the conductive viaconnected to the upper electrode 9 is unnecessary in the manufacturingprocess.

[Method of Manufacturing the Memory Device]

Next, the method of manufacturing the memory device 100 according to thefirst embodiment will be explained in order with respect to the processflow with reference to FIG. 2 and FIG. 3. FIG. 2 is a process drawingshowing an example of the method of manufacturing the memory deviceaccording to the first embodiment, including: (a) a drawing showing aprocess for stacking material for the lower electrode, the variableresistance layer, and the upper electrode above the first insulatinglayer; (b) a drawing showing a process for forming the variableresistance element on the first insulating layer; and (c) a drawingshowing a process for forming the interlayer insulating layer to coverthe variable resistance element. FIG. 3 is a process drawing showing anexample of the method of manufacturing the memory device according tothe first embodiment, including: (a) a drawing showing a process forplanarizing the surface of the interlayer insulating layer; (b) adrawing showing a process for exposing the upper electrode included inthe variable resistance element; (c) a drawing showing a process forforming the lead-out line which is in direct contact with the upperelectrode included in the variable resistance element: and (d) a drawingshowing a process for forming the metal line which is connected to theconductive via.

It is to be noted that, as previously stated, a major difference betweenthe first embodiment and a conventional structure is that the lead-outline 12 is in direct contact with the upper electrode 9 included in thevariable resistance element 10. As such, the following description willdetail the lead-out line 12 forming process and omit commonmanufacturing methods used in, for example, the select transistor 6forming process, and silicon semiconductor manufacturing processes.

As shown in FIG. 2 (a), the substrate 1 having a semiconductor whichincludes the select transistor 6 fabricated therein is prepared. Theoutermost surface of the substrate 1 is covered by the insulating layer14 comprising an insulating material such as a silicon oxide (SiO₂). Theconductive via 50 a is formed on the substrate 1 to electrically connectthe lower electrode 7 included in the variable resistance element 10with the drain region 3. The conductive via 50 a is formed by firstforming a contact hole in a predetermined region of the first insulatinglayer 14 which corresponds to the area where the lower electrode 7included in the variable resistance element 10 is formed, then fillingthe contact hole with an electrically conductive material.

Next, as shown in FIG. 2 (a), the materials used to form the lowerelectrode 7, the variable resistance layer 8, and the upper electrode 9are deposited in this order by sputtering. Subsequently, the lowerelectrode 7, the variable resistance layer 8, and the upper electrode 9are processed together by dry etching, forming the variable resistanceelement 10 as shown in FIG. 2 (b). The upper electrode 9, for example,is formed using one or a plurality of the following: gold (Au), platinum(Pt), iridium (Ir), palladium (Pd), silver (Ag), and Rhodium (Rh). Thevariable resistance layer 8 is formed, for example, having a stackedstructure and comprising: TaO_(x) (where 0<x<2.5) and TaO_(y) (wherex<y), HfO_(x) (where 0<x<2.0) and HfO_(y) (where x<y), or ZrO_(x) (where0<x<2.0) and ZrO_(y) (where x<y). The lower electrode 7 has a stackedstructure comprising a material which is electrically conductive. Forexample, the lower electrode 7 is formed from, in order from the layerin contact with the variable resistance layer 8, platinum (Pt), aluminumtitanium nitride (TiAlN), and titanium nitride (TiN). It is to be notedthat the material included in the lower electrode 7, the variableresistance layer 8, and the upper electrode 9 are not limited to theexamples listed above. Moreover, the deposition method of the lowerelectrode 7, the variable resistance layer 8, and the upper electrode 9is not limited to sputtering; chemical vapor deposition (CVD) or adifferent method is also acceptable.

Next, as shown in FIG. 2 (c), the interlayer insulating layer 15comprising for example SiO₂ is deposited to cover the variableresistance element 10. Afterwards, as is shown in FIG. 3 (a),chemical-mechanical polishing (CMP) method is used for polishing andplanarizing the surface of the interlayer insulating layer 15. In thisprocess, it is preferable to control the amount of polish so as not toexpose the variable resistance element 10 while planarizing the surfaceof the interlayer insulating layer 15. If the variable resistanceelement 10 becomes exposed while polishing the interlayer insulatinglayer 15, the shape of the variable resistance element 10 itself willbecome abnormal due to the polishing, or an abnormality with regard tothe CMP equipment for insulation film such as SiO₂ will occur due to thepolishing of a foreign material (the upper electrode 9 or variableresistance layer 8) in the equipment. The amount of polishing iscontrolled to avoid such circumstances. It is to be noted thatplanarizing the surface of the interlayer insulating layer 15 is alsopossible by what is called a resist etch-back method which includescoating the interlayer insulating layer 15 with photoresist andplanarizing the surface, then, under a dry etching condition in whichthe etching speed of the resist and the etching speed of the interlayerinsulating layer 15 are equal, etching in order from the surface,thereby definitively planarizing the surface of the interlayerinsulating layer 15.

Next, as shown in FIG. 3( b), the interlayer insulating layer 15 isetched in order from the surface by what is called an etch-back method,and the etch-back process is stopped at the point in time the upperelectrode 9 included in the variable resistance element 10 is exposed.As a result, only the upper electrode 9 included in the variableresistance element 10 is exposed to the outermost surface of theinterlayer insulating layer 15. That is, apart from the region of theexposed the upper electrode 9, the entire outermost surface is comprisedof an insulating material such as SiO₂.

Also, when the resist etch-back method is used for the planarization ofthe surface of the interlayer insulating layer 15, the etch-back processmay be extended until the upper electrode 9 included in the variableresistance element 10 is exposed, whereupon the etch-back process isstopped. However, while the film thickness of the interlayer insulatinglayer 15 is usually approximately a few hundred nm to 1 micrometer (μm),the film thickness of the upper electrode 9 is comparatively thin atapproximately a few dozen nm to 100 nm. It is therefore necessary whenperforming etch-back to expose the upper electrode 9 included in thevariable resistance element 10 to select etching conditions (such asdegree of vacuum, power, and type of gas) in which the etching amountcan be finely controlled. This is because if the lower electrode 7 isexposed in the etch-back process, the upper electrode 9 and the lowerelectrode 7 will short circuit as a result of the formation of thelead-out line 12.

It is to be noted that, continuing the etch-back process until the upperelectrode 9 is exposed means that the surface of the upper electrode 9will also be etched in the etch-back process. However, because theetch-back process in this manufacturing process is low aspect ratioetching, a high density plasma and such for high aspect ratio etchingsuch as dry etching of a fine contact hole is unnecessary, and is arelatively low power etching process. For this reason, film loss of theupper electrode 9 (particularly when the upper electrode 9 comprises anoble metal) from the etch-back process is negligibly small.

Afterwards, the material included in the lead-out line 12 such asaluminum (Al), is deposited by, for example, a sputtering method. Asshown in FIG. 3 (c), this material is sputtered to form the lead-outline 12 which is in direct contact with the upper electrode 9 includedin the variable resistance element 10. Next, the second insulating layer16 comprising for example SiO₂ is formed on the interlayer insulatinglayer 15 to cover the lead-out line 12, and the surface of the secondinsulating layer 16 is planarized. Subsequently, the contact hole isformed in a predetermined region of the first insulating layer 14, theinterlayer insulating layer 15, and the second insulating layer 16 whichcorrespond to the area where in the source region 2 (or a differentarea) is formed. The conductive via 50 b is then formed by filling thecontact hole with an electrically conductive material such as W, forexample. Furthermore, as shown in FIG. 3 (d), the memory device 100 isrealized upon forming the metal line 13 on the second insulating layer16.

It is possible to use an electrically conductive material such as Al forthe lead-out line 12. Moreover, the lead-out line 12 may also have astacked structure comprising a material which is electricallyconductive. Specifically, when a material having a high electromigrationhardness such as the transition metals chromium (Cr), molybdenum (Mo),niobium (Nb), tantalum (Ta), tungsten (W), titanium (Ti), vanadium (V),zirconium (Zr), or hafnium (Hf), the chemical compounds silicide,nitride, carbide, or boride is used for the portion of the lead-out line12 in direct contact with the upper electrode 9 included in the variableresistance element 10, a sufficiently great current can flow to thevariable resistance element 10, and the degree of freedom can beincreased with respect to the selection of material used in the variableresistance element 10 and to the operation setting values of thevariable resistance element 10 (and the memory device 100). Furthermore,depositing a layer comprising a transition metal such as those describedabove or a chemical compound such as silicide, nitride, carbide, orboride between the lead-out line 12 and the second insulating layer 16,and between the upper electrode 9 and the lead-out line 12 isadvantageous in that the layer also functions as an adhesion layer.

Moreover, the metal line 13 may comprise the same electricallyconductive material as the lead-out line 12 such as Al, and may alsohave a stacked structure comprising a material which is electricallyconductive.

[Configuration of a Nonvolatile Memory Device]

FIG. 4 is a block diagram showing a configuration of a nonvolatilememory device 500 according to the first embodiment.

As shown in FIG. 4, the nonvolatile memory device 500 is includes amemory main body 201 on a substrate having a semiconductor. The memorymain body 201 includes a memory cell array 202 having a plurality of1T1R memory cells arranged in a matrix, a row selection circuit 208, arow driver 207 including word line drivers WLD and source line driversSLD, a column selection circuit 203, a write circuit 206 for writinginformation, a sense amplifier 204 which detects an amount of currentflowing in a selected bit line and determines whether the data indicates“1” or “0”, and a data input and output circuit 205 which performs aninput and output process of input and output data via a DQ terminal. Itis to be noted that the plurality of 1T1R memory cells included in thememory cell array 202 and arranged in a matrix use the memory device 100described in FIG. 1.

Moreover, when the memory cell array 202 uses the variable resistanceelement 10 having a stacked structure including two types ofoxygen-deficient variable resistance layers (a high-concentration oxidelayer and a low-concentration oxide layer), four differentconfigurations with respect to the relative position of the variableresistance element 10 and the transistor (which one is connected to thebit line) and the arrangement of the source line (parallel to the bitline or to the word line) can be employed, and are explained in (1)through (4) below. Below, the nonvolatile memory device having a 1T1Rmemory cell array according to (2) is used in the explanation withreference to FIG. 4, but explanation of the configurations of the 1T1Rmemory cell array according to (1), (2), (3), or (4) is possible all thesame.

-   (1) The bit line is connected to the high-concentration oxide layer    included in the variable resistance element 10, the source line is    connected to the transistor 6, and the source line is arranged to be    parallel to the bit line.-   (2) The bit line is connected to the high-concentration oxide layer    included in the variable resistance element 10, the source line is    connected to the transistor 6, and the source line is arranged to be    parallel to the word line.-   (3) The source line is connected to the high-concentration oxide    layer included in the variable resistance element 10, the bit line    is connected to the transistor 6, and the source line is arranged to    be parallel to the bit line.-   (4) The source line is connected to the high-concentration oxide    layer included in the variable resistance element 10, the bit line    is connected to the transistor 6, and the source line is arranged to    be parallel to the word line.

The nonvolatile memory device 500 in FIG. 4 includes, as a write powersource 211, a power source for low resistance (LR) writing 212(hereinafter referred to as LR power source 212) and a power source forhigh resistance (HR) writing 213 (hereinafter referred to as HR powersource 213). Here, LR writing refers to changing the resistance state ofthe variable resistance element from a high resistance state to a lowresistance state, and HR writing refers to changing the resistance stateof the variable resistance element from a low resistance state to a highresistance state. An output V2 of the LR power source 212 is supplied tothe row driver 207, and an output V1 of the HR power source 213 issupplied to the write circuit 206.

Furthermore, the nonvolatile memory device 500 includes an address inputcircuit 209 which receives an address signal input from an externalsource, and a control circuit 210 which controls operation of the memorymain body 201 based on a control signal input from an external source.

The memory cell array 202 includes, formed above the substrate having asemiconductor: a plurality of word lines WL0, WL1, WL2 . . . and bitlines BL0, BL1, BL2 . . . arranged in an array and crossing each other;source lines SL0, SL2 . . . deposited between the word lines WL0, WL1,WL2 . . . ; a plurality of NMOS transistors N11, N12, N13, N21, N22,N23, N31, N32, N33 . . . (hereinafter referred to as transistors N11,N12 . . . ) each deposited to a corresponding crosspoint of the wordlines WL0, WL1, WL2 . . . and bit lines BL0, BL1, BL2 . . . ; and aplurality of variable resistance elements R11, R12, R13, R21, R22, R23,R31, R32, R33 . . . (hereinafter referred to as variable resistanceelements R11, R12 . . . ) connected in serial in a one-to-onerelationship with the transistors N11, N12 . . . . The word lines,source lines, bit lines, NMOS transistors, and variable resistanceelement included in the memory cell array 202 are arranged in twodimensions, that is, lateral and longitudinal directions as is shown inFIG. 4, but only a portion of the memory cell array 202 is depicted inFIG. 4 for ease of explanation. A plurality of 1T1R memory cells M11,M12, M13, M21, M22, M23, M31, M32, M33 . . . (hereinafter referred to asmemory cells M11, M12 . . . ) are arranged in a matrix, and each of thememory cells M11, M12 . . . is configured from a corresponding one ofthe word lines WL0, WL1, WL2 . . . , a corresponding one of the bitlines BL0, BL1, BL2 . . . , a corresponding one of the source lines SL0,SL2 . . . , a corresponding one of the transistors N11, N12 . . . , anda corresponding one of the variable resistance elements R11, R12 . . . .

As shown in FIG. 4, gates of the transistors N11, N21, N31 . . . areconnected to the word line WL0, gates of the transistors N12, N22, N32 .. . are connected to the word line WL1, and gates of the transistorsN13, N23, N33 . . . are connected to the word line WL2. Furthermore, thetransistors N11, N21, N31 . . . and the transistors N12, N22, N32 . . .are connected in common to the source line SL0, and the transistors N13,N23, N33 . . . and the transistors N14, N24, N34 . . . are connected incommon to the source line SL2.

Moreover, one terminal of each of the variable resistance elements R11,R12, R13 . . . is connected to the bit line BL0, and the other terminalof each of the variable resistance elements R21, R22, R23 . . . isconnected to the bit line BL1. In the same manner, one terminal of eachof the variable resistance elements R31, R32, R33 . . . is connected tothe bit line BL2.

The address input circuit 209 receives an address signal from anexternal circuit (not shown in the drawings), and, based on this addresssignal, simultaneously outputs a row address signal to the row selectioncircuit 208 and a column address to the column selection circuit 203.Here, the address signal is a signal indicating an address of a specificmemory cell to be selected from among the plurality of memory cells M11,M12 . . . . Moreover, the row address signal is a signal indicating anaddress of a row which is part of the address indicated by the addresssignal, and the column address is a signal which is part of the addressindicated by the address signal.

In a data write cycle, the control circuit 210 outputs, to the writecircuit 206, a write command signal instructing application of a voltagefor writing, according to input data Din input into the data input andoutput circuit 205. On the other hand, in a data read cycle, the controlcircuit 210 outputs, to the sense amplifier 204, a read command signalinstructing a read operation.

The row selection circuit 208 receives the row address signal outputfrom the address input circuit 209, and selects one of the word linesWL0, WL1, WL2 . . . according to the row address signal. The row driver207 applies a predetermined voltage to the word line selected by the rowselection circuit 208, based on an output signal of the row selectioncircuit 208.

Similarly, the row selection circuit 208 receives the row address signaloutput from the address input circuit 209, and selects one of the sourcelines SL0, SL2 . . . according to the row address signal. The row driver207 applies a predetermined voltage to the source line selected by therow selection circuit 208, based on an output signal of the rowselection circuit 208. Furthermore, the column selection circuit 203receives the column address signal output from the address input circuit209, selects one of the bit lines BL0, BL1, BL2 . . . according to thecolumn address signal, and applies a voltage for writing or a voltagefor reading to the selected bit line.

The write circuit 206 outputs, to the column selection circuit 203, asignal instructing the application of the voltage for writing to theselected bit line when the write circuit 206 receives the write commandsignal output from the control circuit 210. Moreover, in the data readcycle, the sense amplifier 204 detects an amount of current flowing inthe selected bit line to be read, and determines whether the dataindicates “1” or “0”. The resultant output data DO is output to theexternal circuit via the data input and output circuit 205.

FIG. 5 is a cross-sectional SEM photograph of the variable resistanceelement 10 experimentally manufactured using the method of manufacturingthe memory device 100 according to the first embodiment. As shown inFIG. 5, the variable resistance element 10 includes the lower electrode7, the variable resistance layer 8, and the upper electrode 9.

The lower electrode 7 has a stacked structure comprising a materialwhich is electrically conductive. Specifically, the lower electrode 7has a stacked structure comprising, in order from the layer in contactwith the variable resistance layer 8, platinum (Pt), aluminum titaniumnitride (TiAlN), and titanium nitride (TiN). It is to be noted that,according to the first embodiment, the lower electrode 7 is not limitedto this stacked structure.

Moreover, in FIG. 5, the lead-out line 12 also has a stacked structurecomprising Pt and TiAlN. Specifically, the layer which is in directcontact with the upper electrode 9 comprises TiAlN. The lead-out line 12is also not limited to this stacked structure, and may be a single layercomprising an electrically conductive material such as Al as previouslystated, or have a stacked structure comprising a material which iselectrically conductive. Specifically, when a material having a highelectromigration hardness such as the transition metals chromium (Cr),molybdenum (Mo), niobium (Nb), tantalum (Ta), tungsten (W), titanium(Ti), vanadium (V), zirconium (Zr), or hafnium (Hf), the chemicalcompounds silicide, nitride, carbide, or boride is used for the portionof the lead-out line 12 in direct contact with the upper electrode 9included in the variable resistance element 10, a sufficiently greatcurrent can flow to the variable resistance element 10. Accordingly, thedegree of freedom can be increased with respect to the selection ofmaterial used in the variable resistance element 10 and to the operationsetting values of the variable resistance element 10 (and the memorydevice 100).

Furthermore, as is shown in FIG. 5, the lead-out line 12 is in directcontact with and completely covers the upper electrode 9 included in thevariable resistance element 10, and a gap is not formed between thelead-out line 12 or the interlayer insulating layer 15 (interlayerinsulating film) and, for example as is used in FIG. 5, silicon oxide(SiO₂). This indicates that the layer (TiAlN) included in the lead-outline 12 having a stacked structure and in direct contact with theinterlayer insulating layer 15 and the upper electrode 9 included in thevariable resistance element 10 functions as an adhesion layer.

As is shown in FIG. 5, when the area of the portion of the upperelectrode 9, which is included in the variable resistance element 10,and the lead-out line 12 that is electrically connected is approximatelyequal to the area of the upper electrode 9, as previously stated, anamount of current can flow to the variable resistance element 10 that issufficiently greater than in a conventional configuration (when aconductive via is used). Accordingly, the degree of freedom can beincreased with respect to the selection of material used in the variableresistance element 10 and to the operation setting values of thevariable resistance element 10 (and the memory device 100). Moreover,because a photomask which defines the conductive via connected to theupper electrode 9 is unnecessary, manufacturing costs and the number ofmanufacturing processes can be reduced.

It is to be noted that, while a small amount of film thicknessunevenness of the upper electrode 9 can be seen in FIG. 5, thisunevenness forms when the variable resistance element 10 is formed bydry etching (see FIG. 2 (b)). With the proper dry etching processconditions, film thickness unevenness of the upper electrode 9 can befurther reduced.

With the memory device 100 according to the first embodiment asdescribed above, the physical connection shape (contact shape) betweenthe electrically connected the lead-out line 12 and the upper electrode9 included in the variable resistance element 10 can be improved, andfilm thickness unevenness of the upper electrode 9 can be reduced. Inother words, in the memory device according to the first embodiment,because a conductive via does not need to be placed between the upperelectrode 9 and the lead-out line 12, the shape abnormality formed atthe interface between the upper electrode 9 and the conductive via, aswell as the film thickness unevenness of the upper electrode 9 as aresult of over etching at the time the contact hole is formed in orderto form the conductive via can be reduced. As a result, it is possibleto provide the memory device 100 having the variable resistance element10 in which initial resistance value fluctuations are suppressed.

(Second embodiment)

[Configuration of the Memory Device]

A memory device 200 according to the second embodiment is different fromthe memory device 100 according to the first embodiment in that themetal line 17 which is not connected to the upper electrode 9, is formedat the same time as the lead-out line 12. Other configurations are thesame as the memory device 100 according to the first embodiment, and assuch, descriptions thereof will be omitted.

FIG. 6 is a diagrammatic view of the memory device 200 according to thesecond embodiment. FIG. 6 (a) is a cross-sectional view showing aconfiguration of the memory device, FIG. 6 (b) is a cross-sectional viewas viewed from the direction of the arrow along the line B-B′ shown inFIG. 6 (a), and FIG. 6 (c) is a plain view as viewed from the surface ofFIG. 6 (a). It is to be noted that in FIG. 6 (c), a number ofconstituents of the memory device which cannot be seen from the surfaceare shown with dashed lines and labeled with reference numerals. Thereference numerals used in FIG. 6 are the same as those used for theconstituents in FIG. 1 through FIG. 3, and as such, descriptions thereofwill be omitted.

The memory device 200 is provided with a plurality of 1T1R memory cells.Specifically, as shown in FIG. 6 (a), each of the memory cells is formedfrom the select transistor 6 and the variable resistance element 10connected in serial via the conductive via 50 a. The select transistor 6includes, manufactured above a substrate 1 having a semiconductor, thesource region 2, the drain region 3, the gate oxide film 4, and the gateelectrode 5 formed on the gate oxide film 4.

According to the second embodiment, the variable resistance element 10has a cylindrical shape. In other words, the variable resistance element10 has a cylindrical shape when viewed from above, as is shown in FIG. 6(c). It should be noted that the variable resistance element 10 is notlimited to this shape, and may be formed into arectangular-parallelepiped-shape, for example (in this case, thevariable resistance element will have a tetragonal shape when viewedfrom above like in FIG. 6 (c)).

The variable resistance element 10 includes the variable resistancelayer 8 which is disposed between the lower electrode 7 and the upperelectrode 9 and whose resistance value changes in response to an appliedvoltage. Here, the lower electrode 7 and the drain region 3 areelectrically connected via the conductive via 50 a. Moreover, the gateelectrode 5 is connected to a word line (not shown in the drawings).

Additionally, the interlayer insulating layer 15 is deposited in thevicinity of the variable resistance element 10. A lead-out line 12including a noble metal is deposited on the upper portion of the upperelectrode 9 included in the variable resistance element 10 so as tocover the upper electrode 9. As shown in FIG. 6 (b), the lead-out line12 is provided in common to a plurality of the memory cells.

Furthermore, as shown in FIG. 6 (a), the metal line 17 is deposited onthe interlayer insulating layer 15 in the area above a diffusion layer18. The diffusion layer 18 is connected to the metal line 17 via aconductive via 50 c. As shown in FIG. 6 (c), the metal line 17 isprovided in common to a plurality of the memory cells. The gateelectrode 5 of the select transistor 6 included in the memory cell isconnected to the word line (not shown in the drawings), and the upperelectrode 9 included in the variable resistance element 10 or one of thediffusion layer 18 of the transistor (for example, the source region 2)is connected to a bit line (not shown in the drawings). As shown in FIG.4, the word lines and the bit lines are disposed so as to cross eachother. A plurality of the memory cells are disposed in an array in thedirection of the word lines and the bit lines, forming a memory cellarray. In other words, in the variable resistance element 10 accordingto the first embodiment, the second insulating layer 16 comprising forexample SiO₂ is formed above the lead-out line 12 and the interlayerinsulating layer 15, and the metal line 13 is further formed above thesecond insulating layer 16, but in the variable resistance element 10according to the second embodiment, the lead-out line 12 and the metalline 17 are both formed above the interlayer insulating layer 15 at thesame time and deposited in the same plane.

With the above configuration, it is possible for the memory device 200according to the second embodiment to have fewer metal line layers thanthe first embodiment.

[Method of Manufacturing the Memory Device]

Next, the method of manufacturing the memory device 200 according to thesecond embodiment will be explained in order with respect to the processflow with reference to FIG. 7. FIG. 7 is a process drawing showing anexample of the method of manufacturing the memory device according tothe second embodiment, including: (a) a drawing showing a process forplanarizing the surface of the interlayer insulating layer; (b) adrawing showing a process for forming the conductive via which isconnected to the metal line; (c) a drawing showing a process forexposing the upper electrode included in the variable resistanceelement; and (d) a drawing showing a process for forming the metal linewhich is connected to the conductive via at the same time as theelectrode line which is in direct contact with the upper electrode. Itis to be noted that, the process flow of the second embodiment is thesame as the first embodiment up until the planarization of the surfaceof the interlayer insulating layer 15 which exposes the variableresistance element 10 (corresponding to FIG. 3 (a)), and as such, thosedescriptions will be omitted, and the process flow thereafter will beexplained.

As shown in FIG. 7 (a), after planarizing the interlayer insulatinglayer 15 exposing the variable resistance element 10, the conductive via50 c for connecting the diffusion layer 18 and the metal line 17 isformed. Here, the conductive via 50 c is formed for connecting thediffusion layer 18 as an example, but it may be a conductive via forconnecting a different layer.

As shown in FIG. 7 (b), first a contact hole is formed for theconductive via 50 c by dry etching. Afterwards, the contact hole isfilled with tungsten (W) by CVD. Next, the surface of the W is removedby CMP, forming the conductive via 50 c filled with W.

Next, as shown in FIG. 7 (c), the interlayer insulating layer 15 isetched in order from the surface by what is called an etch-back method,and the etch-back process is stopped at the point in time the upperelectrode 9 included in the variable resistance element 10 is exposed.As a result, the upper electrode 9, which is included in the variableresistance element 10, and the conductive via 50 c are exposed to theoutermost surface of the interlayer insulating layer 15. Apart from theregion of the exposed the upper electrode 9 and the conductive via 50 c,the entire outermost surface is comprised of an insulating material suchas SiO₂.

Also, when the resist etch-back method is used for the planarization ofthe surface of the interlayer insulating layer 15, the etch-back processmay be extended until the upper electrode 9 included in the variableresistance element 10 is exposed, whereupon the etch-back process isstopped. However, while the film thickness of the interlayer insulatinglayer 15 is usually approximately a few hundred nm to 1 micrometer (μm),the film thickness of the upper electrode 9 is comparatively thin atapproximately a few dozen nm to 100 nm. It is therefore necessary whenperforming etch-back to expose the upper electrode 9 included in thevariable resistance element 10 to select etching conditions (such asdegree of vacuum, power, and type of gas) by which the etching amountcan be finely controlled. If the lower electrode 7 is exposed in theetch-back process, the upper electrode 9 and the lower electrode 7 willshort circuit as a result of the formation of the lead-out line 12.

It is to be noted that, continuing the etch-back process until the upperelectrode 9 is exposed means that the surface of the upper electrode 9will also be etched in the etch-back process. However, because theetch-back process in this manufacturing process is low aspect ratioetching, a high density plasma and such for high aspect ratio etchingsuch as dry etching of a fine contact hole is unnecessary, and is arelatively low power etching process. For this reason, film loss of theupper electrode 9 (particularly when the upper electrode 9 comprises anoble metal) from the etch-back process is negligibly small.

Afterwards, the material included in the lead-out line 12 and the metalline 17 such as Al is deposited by, for example, a sputtering method. Bysputtering this material to simultaneously form, as is shown in FIG. 7(d), the lead-out line 12 which is in direct contact with the upperelectrode 9 included in the variable resistance element 10, and themetal line 17 which is connected to the conductive via 50 c, the memorydevice 200 can be realized.

In the second embodiment, the upper electrode 9 comprises, but is notlimited to, for example, one or a combination of the materials: Au, Pt,Ir, Pd, Ag, and Cu; the variable resistance layer 8 is formed, forexample, having a stacked structure and comprising, but not limited to,for example: TaO_(x) (where 0<x<2.5) and TaO_(y) (where x<y), HfO_(x)(where 0<x<2.0) and HfO_(y) (where x<y), or ZrO_(x) (where 0<x<2.0) andZrO_(y) (where x<y). Moreover, the deposition method of the lowerelectrode 7, the variable resistance layer 8, and the upper electrode 9is not limited to sputtering; CVD or a different method is alsoacceptable.

It is possible to use an electrically conductive material such as Al forthe lead-out line 12 and the metal line 17. Moreover, the lead-out line12 and the metal line 17 may also have a stacked structure comprising amaterial which is electrically conductive. Specifically, when a materialhaving a high electromigration hardness such as the transition metalsCr, Mo, Nb, Ta, W, Ti, V, Zr, or Hf, the chemical compounds silicide,nitride, carbide, or boride is used for the portion of the lead-out line12 in direct contact with the upper electrode 9 included in the variableresistance element 10, a sufficiently great current can flow to thevariable resistance element 10, and the degree of freedom can beincreased with respect to the selection of material used in the variableresistance element 10 and to the operation setting values of thevariable resistance element 10 (and the memory device 200) Furthermore,the layer of the lead-out line comprising a transition metal such asthose described above or a chemical compound such as silicide, nitride,carbide, or boride is provided to interface with the second insulatinglayer 16 or the upper electrode 9 and thereby advantageously functionsas an adhesion layer.

By using a plurality of 1T1R memory cells configured in a memory cellarray and arranged in a matrix, the memory device 200 according to thesecond embodiment formed as described above can be configured as anonvolatile memory device in the same manner as the nonvolatile memorydevice 500 according to the first embodiment previously described, andcan function successfully in the same manner as the first embodiment.

(Third embodiment)

[Configuration of the Memory Device]

A memory device 300 according to the third embodiment is different fromthe memory device 100 according to the first embodiment in that thelead-out line 12 is in contact with the variable resistance layer 8 inaddition to the upper electrode 9 which are included in the variableresistance element 10. Other configurations are the same as the memorydevice 100 according to the first embodiment, and as such, descriptionsthereof will be omitted.

FIG. 8 is a diagrammatic view of the memory device according to thethird embodiment. FIG. 8 (a) is a cross-sectional view of the memorydevice 300. FIG. 8 (b) is a cross-sectional view as viewed from thedirection of the arrow along the line C-C′ shown in FIG. 8 (a). Thereference numerals used in FIG. 8 (a) are the same as those used for theconstituents in FIG. 1 through FIG. 3, and as such, descriptions thereofwill be omitted.

The memory device 300 is provided with a plurality of 1T1R memory cells.Specifically, as shown in FIG. 8 (a), each of the memory cells is formedfrom the select transistor 6 and the variable resistance element 10connected in serial via the conductive via 50 a. The select transistor 6includes, manufactured above a substrate 1 having a semiconductor, thesource region 2, the drain region 3, the gate oxide film 4, and the gateelectrode 5 formed on the gate oxide film 4.

The variable resistance element 10 includes the variable resistancelayer 8 which is disposed between the lower electrode 7 and the upperelectrode 9 and whose resistance value changes in response to an appliedvoltage. Here, the lower electrode 7 and the drain region 3 areelectrically connected via the conductive via 50 a. Moreover, the gateelectrode 5 is connected to a word line (not shown in the drawings).

Additionally, the interlayer insulating layer 15 is deposited in thevicinity of the variable resistance element 10. The lead-out line 12 isdeposited on the upper portion of the upper electrode 9 included in thevariable resistance element 10 so as to cover the upper electrode 9. Asshown in FIG. 8 (b), the lead-out line 12 is provided in common to aplurality of the memory cells. Moreover, the bottommost surface of thebottom surfaces of the lead-out line 12 is formed to be lower than thetop surface of the upper electrode 9. In other words, the lead-out line12 is formed to cover the entire surface and the walls of the upperelectrode 9 as well as a portion of the side walls of the variableresistance layer 8, and furthermore is formed on the interlayerinsulating layer 15 within a predetermined range from the variableresistance element 10. It is to be noted that the lead-out line 12 mayalso be formed to cover a portion of the upper electrode 9 and a portionof the side walls of the upper electrode 9 and the variable resistancelayer 8, and not the entire top surface of the upper electrode 9, andmay be formed on a portion of the interlayer insulating layer 15 withina predetermined range from the upper electrode 9.

Furthermore, the second insulating layer 16 comprising for example SiO₂is disposed on the lead-out line 12 and the interlayer insulating layer15. The metal line 13 is disposed on the second insulating layer 16. Themetal line 13 is provided in common to a plurality of the memory cells.The source region 2 included in the select transistor 6 is connected,via a conductive via 50 b, to the diffusion layer serving as a sourceline, or the metal line 13. The gate electrode 5 of the selecttransistor 6 included in the memory cell is connected to the word line(not shown in the drawings), and the upper electrode 9 included in thevariable resistance element 10 or one portion of the diffusion layer 18of the transistor (for example, the source region 2) is connected to abit line (not shown in the drawings). As shown in FIG. 4, the word linesand the bit lines are disposed so as to cross each other. The memorycells are disposed in an array in the direction of the word lines andthe bit lines, forming a memory cell array.

With the above configuration, there is an advantage in which theetch-back margin can be increased in the forming process of the lead-outline 12, that is, the etch-back process which exposes the upperelectrode 9.

[Method of Manufacturing the Memory Device]

The method of manufacturing the memory device 300 according to the thirdembodiment that differs from the method of manufacturing the memorydevice 100 according to the first embodiment in that it includes amanufacturing process in which the interlayer insulating layer 15 isetched in order from the surface by what is called an etch-back method,and the etch-back process is stopped at the point in time at least aportion of the upper electrode 9 and the variable resistance layer 8included in the variable resistance element 10 are exposed.

FIG. 9 is a process drawing showing an example of the method ofmanufacturing the memory device according to the third embodiment inwhich the process of exposing the variable resistance layer and theupper electrode included in the variable resistance element is shown (across-sectional diagrammatic view after completion of the etch-backprocess). Especially when the variable resistance layer 8 comprises anoxide such as TaO_(x) (where 0<x<2.5), HfO_(x) (where 0<x<2.0) andHfO_(y), or ZrO_(x) (where 0<x<2.0), the portion of the variableresistance layer 8 in contact with the interlayer insulating layer 15comprising for example SiO₂ is strongly oxidized to a further degree inthe process depositing the interlayer insulating layer 15 to cover thevariable resistance element 10 (in the first embodiment, correspondingto FIG. 2 (c)), whereby the resistance is increased (the variableresistance layer becomes insulative). For this reason, even if thevariable resistance layer 8 is exposed in the etch-back process andbrought into contact with the lead-out line 12, current will not flowfrom the portion in contact. Current flows to or from the lead-out line12 via the upper electrode 9 only. Consequently, an electricalcharacteristic of the variable resistance element 10 is basically thesame as that of the first embodiment.

From the viewpoint of the manufacturing method, in the etch-back processof the interlayer insulating layer 15, with the third embodiment, theelectrical characteristic will not change even if the variableresistance element 10 is exposed to the variable resistance layer 8,unlike with the first embodiment in which only the upper electrode 9included in the variable resistance element 10 is to be exposed. Thatis, the memory cell array may contain a mixture of the memory cellsaccording to the first embodiment (FIG. 1) and the memory cellsaccording to the third embodiment (FIG. 8). For this reason, using thethird embodiment is advantageous in that the etch-back margin can beincreased.

By using a plurality of 1T1R memory cells formed in a memory cell arrayand arranged in a matrix, the memory device 300 according to the thirdembodiment formed as described above can be configured as a nonvolatilememory device 500 in the same manner as the memory device 100 accordingto the first embodiment previously described, and can functionsuccessfully in the same manner as the first embodiment.

(Configuration Variations of the First Through Third Embodiments)

In each of the above embodiments, the transition metal oxide layer whichforms the variable resistance layer is described as having a stackedstructure comprising a tantalum oxide, but achievement of thefunctionality of the present invention as described above, as well asthe present invention itself, is not limited to the use of tantalumoxide. A stacked structure comprising a hafnium (Hf) oxide or a stackedstructure comprising a zirconium (Zr) oxide is also acceptable. Examplesof when a hafnium oxide and a zirconium oxide is used in the variableresistance layer will be explained hereinafter.

Hereinafter use of a variable resistance layer having a stackedstructure comprising hafnium oxide will be described. The variableresistance layer has a stacked structure of a first hafnium oxide layerhaving a low oxygen concentration, and a second hafnium oxide layerhaving a high oxygen concentration. It is preferable that the firsthafnium oxide have a composition that approximately satisfies 0.9≦x≦1.6when expressed as HfO_(x) and the second hafnium oxide have acomposition that approximately satisfies 1.8<y when expressed asHfO_(y), and it is preferable that a film thickness of the secondhafnium oxide be no less than 3 nm and no more than 4 nm.

When hafnium oxide is used, the variable resistance layer is formed asfollows. First, the first hafnium oxide layer is formed on the lowerelectrode through reactive sputtering in argon gas and oxygen gas usinga target of Hf. After the first hafnium oxide layer is formed, thesecond hafnium oxide layer is formed by exposing the surface of thefirst hafnium oxide layer to a plasma of argon and oxygen gas. Same aswhen tantalum oxide is used, the oxygen content atomic percentage of thefirst hafnium oxide layer can easily be adjusted by changing the flowrate of the oxygen gas with respect to the argon gas in the reactivesputtering. It is to be noted that a substrate does not especially needto be heated; room temperature is acceptable.

Moreover, the film thickness of the second hafnium oxide layer caneasily be adjusted by changing the exposure time to the plasma of argonand oxygen gas. When the composition of the first hafnium oxide layerexpressed as HfO_(x) satisfies 0.9≦x≦1.6, and the composition of thesecond hafnium oxide layer expressed as HfO_(y) satisfies 1.8<y, and thefilm thickness of the second hafnium oxide layer is no less than 3 nmand no more than 4 nm, a stable variable resistance characteristic canbe realized.

Hereinafter use of a variable resistance layer having a stackedstructure comprising zirconium oxide will be described. The variableresistance layer has a stacked structure of a first zirconium oxidelayer having a low oxygen concentration, and a second zirconium oxidelayer having a high oxygen concentration. It is preferable that thefirst zirconium oxide have a composition that approximately satisfies0.9≦x≦1.4 when expressed as ZrO_(x) and the second zirconium oxide havea composition that approximately satisfies 1.9<y when expressed asZrO_(y), and it is preferable that a film thickness of the secondzirconium oxide be no less than 1 nm and no more than 5 nm.

When zirconium oxide is used, the variable resistance layer is formed asfollows. First, the first zirconium oxide layer is formed on the lowerelectrode through reactive sputtering in argon gas and oxygen gas usinga target of Zr. After the first zirconium oxide layer is formed, thesecond zirconium oxide layer is formed by exposing the surface of thefirst zirconium oxide layer to a plasma of argon and oxygen gas. Same aswhen tantalum oxide is used, the oxygen content atomic percentage of thefirst zirconium oxide layer can easily be adjusted by changing the flowrate of the oxygen gas with respect to the argon gas in the reactivesputtering. It is to be noted that a substrate does not especially needto be heated; room temperature is acceptable.

Moreover, the film thickness of the second zirconium oxide layer caneasily be adjusted by changing the exposure time to the plasma of argonand oxygen gas. When the composition of the first zirconium oxide layerexpressed as ZrO_(x) satisfies 0.9≦x≦1.4, and the composition of thesecond zirconium oxide layer expressed as ZrO_(y) satisfies 1.9<y, andthe film thickness of the second zirconium oxide layer is no less than 1nm and no more than 5 nm, a stable variable resistance characteristiccan be realized.

It is to be noted that the present invention is not limited to theabove-described embodiments, and various improvements and modificationsmay be carried out within a scope that does not depart from the essenceof the present invention.

The electrode which is in contact with the tantalum oxide layer can beformed, for example, using a material having a standard electrodepotential higher than a standard electrode potential of tantalum such asone or a plurality of the following: gold (Au), platinum (Pt), iridium(Ir), palladium (Pd), silver (Ag), and Rhodium (Rh), and it ispreferable that the electrode which is in contact with the tantalumoxide layer be formed using a material having a standard electrodepotential lower than the material included in electrode which is not incontact with the tantalum oxide layer (W, Ni, or TaN, for example). Byadopting such a configuration, a resistance changing operation can beperformed stably at the variable resistance layer.

Moreover, the memory device may include a plurality of memory cellshaving a stacked structure of a plurality of layers.

Moreover, the material included in the upper electrode and the lowerelectrode is not limited to the material indicated in the embodimentsdescribed above; other materials may be used.

Moreover, in the embodiments described above, the source line includedin the memory device is arranged to be parallel to the word line, but itis acceptable if the source line is arranged to be parallel to the bitline. Moreover, a configuration is used in which the source line appliesa common potential to the transistor, but a configuration in which asource line selection circuit and a source line driver having the sameconfiguration as the row selection circuit and the row driver,respectively, and in which the selected source line and the unselectedsource line are driven using different voltages (and polarities) is alsoacceptable.

Various modifications of the exemplary embodiment as well as embodimentsresulting from arbitrary combinations of constituent elements ofdifferent exemplary embodiments that may be conceived by those skilledin the art are intended to be included within the scope of the presentinvention as long as these do not depart from the essence of the presentinvention. For example, a memory system such as a ReRAM memory system(nonvolatile memory device) including the memory device according to thepresent invention is included within the scope of the present invention.

[Industrial Applicability]

The memory device according to the present invention is useful as amemory device having variable resistance elements in which initialresistance value fluctuations are suppressed by improving the physicalconnection shape between an electrically connected a line and an upperelectrode of the variable resistance elements, and by reducing the filmthickness unevenness of the upper electrode.

REFERENCE SIGNS LIST

[Description of Embodiments]

-   1 substrate-   2 source region-   3 drain region-   4 gate oxide film-   5 gate electrode-   6 select transistor (transistor)-   7 lower electrode-   8 variable resistance layer-   9 upper electrode-   10 variable resistance element-   12 lead-out line-   13, 17 metal line-   14 first insulating layer-   15 interlayer insulating layer-   16 second insulating layer-   18 diffusion layer-   50 a, 50 b, 50 c conductive via-   100, 200, 300 memory device-   201 memory main body-   202 memory cell array-   203 column selection circuit-   204 sense amplifier-   205 data input and output circuit-   206 write circuit-   207 row driver-   208 row selection circuit-   209 address input circuit-   210 control circuit-   211 write power source-   212 LR power source-   213 HR power source-   500 nonvolatile memory device-   BL0, BL1, BL2 bit line-   N11, N12, N13, N14, N21, N22, N23, N24, N31, N32, N33, N34    transistor-   M11, M12, M13, M14, M21, M22, M23, M24, M31, M32, M33, M34, memory    cell-   SL0, SL2 source line-   R11, R12, R13, R14, R21, R22, R23, R24, R31, R32, R33, R34 variable    resistance element-   WL0, WL1, WL2, WL3 word line

The invention claimed is:
 1. A memory device comprising: a plurality ofmemory cells; and a lead-out line provided in common to said pluralityof memory cells, wherein each of said memory cells includes: atransistor formed above a substrate; and a variable resistance elementwhich includes a lower electrode, an upper electrode comprising a noblemetal, and a variable resistance layer disposed between said lowerelectrode and said upper electrode, said variable resistance layer has aresistance value that changes reversibly in response to an electricpulse that goes through said transistor and is applied between saidlower electrode and said upper electrode, said lead-out line is indirect contact with said upper electrode included in each of said memorycells, and the bottommost surface of the bottom surface of said lead-outline is formed to be lower than the top surface of said upper electrodeand higher than the top surface of said lower electrode, and said upperelectrode is completely covered by said lead-out line.
 2. The memorydevice according to claim 1, wherein said variable resistance layerincludes a first layer comprising an oxygen-deficient first transitionmetal oxide having a composition represented as MO_(x) (where M denotesa transition metal and O denotes oxygen) and a second layer comprising asecond transition metal oxide having a composition represented as MO_(y)(where x<y) formed on the first layer, and the second layer is incontact with said upper electrode.
 3. The memory device according toclaim 1, wherein at least a portion of a side wall of said variableresistance layer is covered by an insulating layer.
 4. The memory deviceaccording to claim 1, wherein said variable resistance layer comprises atransition metal oxide selected from the group consisting of tantalum,hafnium, and zirconium.
 5. The memory device according to claim 1,wherein said lead-out line has a stacked structure comprising anelectrically conductive material, and a portion of said lead-out linethat is in direct contact with said upper electrode comprises anelectrically conductive material including at least one of chromium,molybdenum, niobium, tantalum, tungsten, titanium, vanadium, zirconium,hafnium, silicon, nitrogen, carbon, and boron.
 6. A method ofmanufacturing a memory device including a plurality of memory cells eachof which includes a transistor and a variable resistance element, themethod comprising: forming the transistor above a substrate; forming alower electrode of the variable resistance element to correspond to thetransistor; forming a variable resistance layer on the lower electrode;forming an upper electrode comprising a noble metal on the variableresistance layer; forming an interlayer insulating layer so as to coverthe variable resistance element; exposing at least a portion of a topsurface of the upper electrode; and forming a lead-out line that isprovided in common to the plurality of memory cells so as to be indirect contact with the upper electrode and such that the bottommostsurface of the bottom surface of the lead-out line is lower than the topsurface of the upper electrode and higher than the top surface of thelower electrode, the lead-out line being formed such that the upperelectrode is completely covered by the lead-out line.
 7. The methodaccording to claim 6, comprising: forming, in the variable resistancelayer, a first layer comprising an oxygen-deficient first transitionmetal oxide having a composition represented as MO_(x) (where M denotesa transition metal and O denotes oxygen); and forming, on the firstlayer, a second layer comprising a second transition metal oxide havinga composition represented as MO_(y) (where x<y).
 8. The method accordingto claim 6, wherein said exposing includes planarizing the interlayerinsulating layer, and exposes at least a portion of the upper electrodeon a surface of the interlayer insulating layer after planarizing theinterlayer insulating layer.
 9. The method according to claim 6, whereinin said forming of a lead-out line, the lead-out line and an other lineare formed at the same time.